1. Field of the Invention
This invention pertains to data communication systems that employ a multitude of carriers for conveying information through a transmission medium. In particular, this invention addresses the problem of peak to average power ratio reduction in such systems.
2. Description of the Background Art
A multicarrier transmission system is one that employs Frequency Division Multiplexed (FDM) subcarriers for transmission of data across a communication channel. A comprehensive description of multicarrier systems is given in by John A. C. Bingham in xe2x80x9cMulticarrier Modulation for Data Transmission: An idea whose time has comexe2x80x9d, IEEE Communication Magazine, Vol. 28, No 5, pp. 5-14, May 1990.
A typical multicarrier transmission system,is depicted in FIG. 1. It shows serial data in the form of bits input to a serial to parallel converter 100, which may also frame the bits. (A frame is an ordered sequence of bits of a given size). The output of block 100 is fed to a Forward Error Correction (FEC) block 200, which adds redundant bits to facilitate correction of possible errors introduced by the transmission medium 900. The output bit frame from the encoder is mapped to symbols in the mapper 300. The frame of symbols then passes through the modulator 400. The modulation could be carried out, for instance, using Inverse Fast Fourier Transform (IFFT) and the corresponding demodulation carried out using Fast Fourier Transform (FFT). The modulated signal is converted to serial data 700 and transmitted on to the channel 900, where it suffers several impairments in addition to being corrupted by noise. At the receiver, the received data is grouped into frames by a serial to parallel converter 1000 and subsequently passed through the demodulator 1100. The demodulated symbols are converted to bits and passed through FEC decoder 1200. The data bits are obtained by passing the decoded bits through a parallel to serial converter 1300.
In practice, the data bits input to the transmitter module are independent and the symbols obtained using the bits-to-symbols mapper are also independent. Whenever the number of subcarriers is large, the modulator output magnitude tends to have a Gaussian distribution, which has an infinite peak to average power ratio. A clipper 800, which has a cutoff based on a predesignated clipping probability, is then used to limit the peak signal power that is transmitted on to the channel. The clipping probability needs to be small in order that the error introduced due to clipping is small. If the predesignated clipping probability is reduced, the peak power input to the channel increases, which may cause saturation in the subsequent amplification stages. A higher peak power requires a larger range of amplifier linearity and leads to higher cost. Thus it becomes necessary to limit the peak power transmitted on to the channel by reducing the clipping level while still maintaining a low clipping probability.
FIG. 1 shows a peak detector 500 at the output of the modulator, which activates a symbol modifier 600 whenever the magnitude of any of the modulator output samples exceeds a predesignated threshold or a set of thresholds. The threshold could be set based on the level of peak power that can be transmitted on to the channel, while the task of the symbol modifier is to modify the symbols being modulated in such a manner that the peak is reduced. Although FIG. 1 shows the symbol modifier correcting the symbols that are being modulated, it is possible to achieve the same effect by correcting the modulated samples.
The symbol modifier needs to fulfill a number of objectives:
1. The clipping probability should remain unaltered when the clipping threshold is lowered.
2. The modifications introduced in the symbols should not cause significant increase in the error rate at the receiver.
3. The symbol modifier should have a low complexity.
4. The Peak to Average power Ratio (PAR) reduction scheme should be transparent to the receiver.
5. The modifications made to the data symbols should not cause an increase in the average power.
6. The loss in data rate caused due to symbol modification should be minimal.
Most of the requirements are conflicting and it may not be possible to meet all of them at the same time. The amount of importance given to different requirements by a particular method decides the extent of PAR reduction achievable by that method.
A scheme using a pre-designated set of subcarriers to reduce the PAR is described by J. Tellado and J. Cioffi in xe2x80x9cPAR Reduction in Multicarrier Transmission Systems (97-367)xe2x80x9d, T1E1.4/97-367 Dec. 8, 1997. The method uses an iterative procedure that aims at reducing the largest peak every iteration. A similar scheme by the same authors is detailed in xe2x80x9cRevisiting DMT""s PAR (98-083)xe2x80x9d, T1E1.4/98-083, Mar. 3, 1998. Here, an iterative procedure tries to reduce all the peaks present in each iteration. Both the methods employ a precomputed peak reduction kernel. The reduction in PAR is obtained at the expense of data carrying subcarriers. It is required that the receiver be aware of the predesignated subcarriers to be used for peak reduction.
The modulo-D scheme described in xe2x80x9cA new approach to PAR control in DMT Systemsxe2x80x9d, NF-83, ITUxe2x80x94T, Study Group 15, 11-14 May 1998, uses an expanded constellation in each subcarrier for PAR reduction. The method uses symbols drawn from larger constellations whenever the original modulation produces large peaks. The method results in no data rate loss. The use of larger constellations results in increased average power and higher intermodulation distortion. The receiver needs to perform a reverse operation to the transmitter action to recover the original data symbols.
A sign inversion method is specified in xe2x80x9cAn Efficient Implementation of PAR reduction method based on subset inversionxe2x80x9d, AB-061r2, ITUxe2x80x94T, Study Group 15, 3-7 August, 1998. The method consists of dividing the set of subcarriers into several subsets and achieving a PAR reduction by inverting the sign of the symbols in some of the subsets. The transmitter uses part of the data carrying bits to indicate to the receiver whether a sign inversion has been applied. This method requires receiver cooperation and results in some data rate loss. An error in a subset inversion indicator bit may result in wrong decoding of many of the data bits associated with that subset.
The scheme specified in xe2x80x9cA. E. Jones, T. A. Wilkinson and S. K. Barton, Block coding scheme for reduction of peak to mean envelope power ratio of multicarrier transmission schemes, Electronic Letters, Vol. 30, No. 25 pp 2098-2099, December 1994xe2x80x9d uses a coding scheme with an inherently low PAR. This is achieved at the expense of increase in the redundancy (extra bits added to the data carrying bits.)
This invention pertains to Peak to Average power Ratio (PAR) reduction in multicarrier communication systems that employ a Forward Error Correction (FEC) coding mechanism. A method and apparatus are provided that computes a subcarrier symbol modification that effects a reduction in PAR of the transmitted data samples. The present invention consists of a peak detection mechanism, a procedure for choosing a subcarrier and a symbol modifier scheme. The peak detector uses a threshold for determining whether the PAR reduction is to be applied. A slightly modified version using a lower sampling rate and a threshold over adjacent samples is also disclosed.
Several methods are described for choosing a subcarrier for symbol modification depending on the location(s) of the peak and the effectiveness of the symbol modification on reducing the PAR with very little loss of coding gain. Specific methods apply to single peaks, two peaks and multiple peaks. The methods for choosing the subcarriers can be applied to any PAR scheme that uses symbol modifications for reducing PAR. The symbol modifier uses the fact that subcarrier errors, introduced rarely, will have a minor effect on the coding gain of the error-correction mechanism used. A tradeoff between extent of PAR reduction and loss in coding gain can be made by modifying the threshold of the peak detector.